The present invention is directed generally to mine detection and more specifically to a method and apparatus for acquiring and displaying a radiometric image of a mine.
The detection of land mines and other ordnance on the battlefield has grown in importance with their increased use, not only for military personnel, but also for civilians after hostilities have ceased. The need for new approaches and sensors to increase the speed and efficiency of methods to clear mines is an issue that must be addressed.
The detection of land mines has been an important part of battlefield management since the first use of mines. Similarly, the detection of unexploded ordnance (UXO) for clean-up of long-used test ranges is of concern. The importance of both has grown with time and the task has not become easier. The long term danger posed by mines and UXO to both military personnel and displaced civilian populations during wars was illustrated most recently by experiences during the Gulf War and continues today around the world.
As a further example, the dangers posed by metal and plastic land mines is an everyday issue facing the United States military and others in regions such as Bosnia-Herzegovina. Techniques currently being used are relatively crude and labor intensive. The difficulty in locating these and other mines and UXO throughout the world begs the development of improved sensors to speed the process of clearing them to reduce casualties, injuries, and other costs.
Sensors in the millimeter wave region of the electromagnetic spectrum (typically from 30 to 300 GHz) have been shown to be capable of detecting metal above and under dry sand. For example, the inventors of the present application co-authored an article entitled, "Passive millimeter wave sensors for detection of buried mines," SPIE Vol. 2496, April 1995, incorporated in its entirety herein by reference. The detection capabilities disclosed point to new possibilities for improving mine detection techniques. Passive millimeter wave sensors (or radiometers) provide day/night operation, have good spatial resolution, good signal-to-noise ratio (SNR) providing good target contrast, and clutter is benign compared to the situation with ground penetrating (GPEN) radar and infrared (IR) sensors.
For greater ground penetration, extension of the spectrum to lower frequencies is desirable. For example, in the detection of buried mines, lower frequencies have better ground penetration capability and are expected to give improved detection of such targets. The reduced fill factor (discussed below) and lower resolution that follows from the use of lower frequencies forces the use of large apertures or detection at shorter ranges.
Further, it has been shown that metal targets such as mines placed above soil, under an open sky, stand out clearly against the soil background when using a millimeter wave radiometer as an imaging device. There are three basic factors that allow this to happen.
First, the sky above has a very low brightness temperature (.ltoreq.40K) which means that it acts like a very cold, low power illuminator that bathes the scene with a very low level of millimeter wave radiation. This occurs at certain spectral windows where emissions from the atmospheric oxygen and water are relatively low (for example, below.apprxeq.20 Ghz, and around 35, 94, and 140 GHz).
Second, the soil typically has a high emissivity (or low reflectivity), and thus emits millimeter wave radiation corresponding to its physical temperature (.apprxeq.295K).
Third, the metal target has low emissivity (or high reflectivity). Thus, its own physical temperature is unimportant in terms of its appearance in the millimeter wave regime. The appearance of the metal target is determined by what the target reflects.
These three factors combine as follows: the metal target reflects the low level radiation from the sky and looks like a "cold" object surrounded by the "hot" soil which is emitting a higher level of millimeter wave radiation. This high contrast (295K-40K=255K), combined with typical modem millimeter wave radiometer sensor temperature resolutions of 1K, allows a high signal-to-noise ratio (SNR).
However, the high contrast described above occurs on a clear day, and cloud cover will raise the effective sky temperature, thereby reducing the contrast of the target with its background. A heavy overcast situation can cause the effective sky temperature to rise to about 200K, but this still leaves plenty of contrast.
Similarly, burying the metal target under soil will reduce the target contrast due to the obvious obscuring nature of the soil, but also because of the hot millimeter wave emissions from the overburden. The amount of contrast reduction will vary with the depth of soil coverage, the type of soil, the amount of water content in the soil, and the temperature of the soil.
Another consideration in determining the target contrast in a mine detector system is the "fill factor," a measure of the extent to which the detector's field of view is filled by the reflective target. The smaller the fill factor, the smaller the target contrast. Remote mine detection, as from an airborne platform, requires a sufficiently high resolution sensor to maintain an adequate fill factor.
The effect of the detection frequency on the ability of radiometric techniques to detect buried metal targets has been demonstrated. For example, the inventors of the present application co-authored an article entitled, "Detection of metal and plastic mines using passive millimeter waves," SPIE Vol. 2765, April 1996, incorporated in its entirety herein by reference. Typically the results are plotted as the absolute value of the radiometric temperature difference .vertline..DELTA.T.vertline. between an infinitely thick sand layer and a sand layer of varying thickness over the metal target, versus the thickness of the sand layer. The general trend is that .vertline..DELTA.T.vertline. increases (the sand appears colder than an infinitely thick sand layer) as the sand layer decreases in thickness, until it reaches a maximum value corresponding to the sky temperature directly reflected off the metal target without any obscuring sand. As the sand layer increases in thickness, .vertline..DELTA.T.vertline. approaches zero (the presence of the metal target is totally masked).
The detectability of targets (larger .vertline..DELTA.T.vertline.) improves with lower frequencies and lower water content. For a given minimum resolvable temperature that a detector can resolve (based on its noise characteristics) and a given water content, a lower frequency allows detection to a greater depth.
On the other hand, a plastic target, given its much lower reflectivity and its transparency to radiation rising from below it, produces a much smaller .vertline..DELTA.T.vertline. than a metal target. To quantify the degree of detectability of metal and plastic mines using radiometric techniques, measurements have been performed using some typical mines under varying conditions and detection frequencies. The above articles demonstrate that metal and plastic mines can be detected both on the surface and under soil with varying moisture content.
Past detection techniques involve mechanically scanning the scene by passing the radiometer over the scene in a set pattern, and dwelling a long time at each spot to get a good enough integration time, and then forming the image after the fact. Such a technique is an organized, systematic way of generating an image. However, such a process is time-consuming and wasteful. For instance, the radiometer spends an equally long time over areas without a mine, as it does over areas with a mine. Such a system scans the scene back and forth in an orderly manner using a precise system for moving the radiometer back and forth. The radiometer was mounted on an azimuth elevation platform which uses a motorized system with gears and an apparatus to move the radiometer back and forth.
Thus, the radiometer was moved back and forth in a fixed pattern. As the radiometer was moved across the scene, the system recorded the data and its position, but no display was immediately generated. Also, the system did not have the capability for the operator to move the head to an area of interest and scan over that area for a longer period of time.
Thus, the above described detection system operated by scanning to the left, moving forward, scanning to the right, and moving forward again and repeating the process over and over. A raster scan image of the scene was generated showing a two-dimensional image indicating a mine buried under the soil.
The detection of inert metal and plastic mines at the surface or under dry sand or soil, and leaves, has been demonstrated at both 12 and 44 GHz. Soil with a high water content drastically affects the detectability of the mines. Also, modeling predicts that better performance is possible at lower frequencies. A need therefore exists for a method and apparatus for acquiring and displaying a radiometric image of a buried mine which allows an operator to concentrate on a particular region of interest.